Control methods are known for actuating the injection valve(s) of an injection system in recurring injection cycles in such a way that in each of these injection cycles the injection valve is opened and closed again in such a way that a pressurized fuel is injected into the internal combustion engine as precisely as possible with a previously determined profile over time of an injection rate.
Since the actual profiles over time of the injection rates and therefore also the respectively actually injected quantities of fuel depend, in particular, on the actual opening times and closing times of the injection valves, the opening times and/or the closing times of the injection valves are measured by means of a sensor element with a control method of the generic type and/or with an injection system of the generic type, and the measured opening times and/or the measured closing times are subsequently taken into account in the actuation of the injection valves in the subsequent injection cycles, in order to monitor and/or reduce control errors of one or more control variables of the injection method. For example, the opening time and also the closing time can be control variables of the control method. The setpoint opening time and/or the setpoint closing time are/is typically determined as a function of an instantaneously required injection quantity and/or as a function of an instantaneously required profile over time of the injection rate.
An injection valve, frequently also referred to as an injector, comprises a closure element which can be configured, for example, as a nozzle needle, and an actuator with which the closure element can be moved between a closed position and an upper stop. If the closure element is in the closed position, the injector is closed and no fuel injection takes place. In the closed position, the closure element is typically seated on a lower stop of the injection valve and closes all the injection openings of the injection valve. In order to open the injector, the closure element is raised from the closed position by means of the actuator, with the result that in this way one, more than one, or each of the injection openings is/are opened and the injection of the fuel takes place through the opened injection openings. The closure element can be raised at maximum up to the specified upper stop which therefore defines a maximum stroke height of the closure element relative to the closed position or to the lower stop. In the text which follows, the opening time is to be understood as that time at which the closure element impacts against the upper stop (attaining of the maximum needle stroke), and the closing time is to be understood as that time at which the closure element impacts in the closed position, that is to say against the lower stop. Further characterizing times of an injection cycle are the time at which the closure element leaves the closed position, and the time at which the closure element leaves the upper stop. (Start of the closing movement given a maximum stroke height). According to their time sequences, these times are denoted by OPP1, OPP2, OPP3 and OPP4.
In order to be able to measure the opening time and/or the closing time (OPP2 and/or OPP4), the injection valve comprises a sensor element in which characteristic signals are triggered by the impacting of the closure element in the closed position or against the upper stop. In order to be able to detect these characteristic signals, which are correspondingly also referred to as opening signals or as closing signals of the sensor element, a signal profile over time of the sensor element is detected and evaluated. This generally involves buffering of the signal profile over time. Generally, known injection systems or injection valves have for this purpose data memories whose memory capacity is, however, typically not sufficient to detect a total profile over time of output signals of the sensor element. However, for this reason or else for other reasons such as, for example, in order to accelerate and/or simplify the subsequent data evaluation, the entire signal profile over time of the sensor element is not stored, and the presence of the characteristic signals is subsequently examined, but instead examination is carried out only of parts of the signal profile over time which are contained in one or more predefined time search windows. In this context, these search windows are selected in such a way that they contain the expected closing time and/or the expected opening time of a given injection cycle.
However, if the actual closing time or the actual opening time is chronologically outside the respective search window for a given injection cycle, it is generally not possible to detect the associated characteristic signals and to acquire the opening time or the closing time. Non-detection of the characteristic signal is also referred to in the text which follows as a loss of the characteristic signal or of the closing signal or of the opening signal or is referred to in brief as a control loss. Such a control loss may lead, for example, to a situation in which the injection system can subsequently only continue to be operated in an emergency running mode.
The problem therefore arises of improving the reliability and the robustness of known control methods and injection systems. In particular, the measurement of the opening and closing times of the injection valves is to be improved.